The present invention relates to a refrigeration plant of the type comprising a compressor, and a condenser and an evaporator with pressure reduction means therebetween and communicating with the compressor through a high pressure, outlet channel and a low pressure, inlet channel, respectively. The compressor is of a rotary, positive displacement type having at least one rotor provided with spiral lobes and intervening grooves. The plant is further provided with an intermediate pressure vessel communicating with the condenser through pressure reduction means and with intermediate port means in the compressor through an intermediate pressure channel. The invention further relates to a rotary machine appropriate for use as a compressor in such a plant.
Plants and compressors of such types are earlier known from U.S. Pat. No. 3,568,466, Brandin et al., and U.S. Pat. No. 3,913,346, Moody et al. The intermediate pressure zone in such plants is used for internal cooling purposes within the plant at a temperature level above that of the evaporator. The main cooling purpose is to precool the liquified refrigerant before the supply thereof to the evaporator which results in a more effective use of the evaporator area so that the dimensions thereof can be minimized for a certain capacity simultaneously as the swept volume of the compressor and thus its dimensions can be reduced correspondingly. Furthermore the power required for recompression of the gaseous refrigerant supplied at the intermediate pressure will be less than that if all the refrigerant were supplied at the evaporator pressure. A second cooling purpose applicable when the compressor is driven by an electrical motor, especially important in hermetic systems and heat pump applications, is to pass the intermediate pressure fluid through the motor in order to guarantee an efficient cooling thereof under all driving conditions.
Even though the description of the compressor for a refrigeration plant in this specification is restricted to the type comprising two intermeshing rotors of male and female type provided with helical lands and intervening grooves the invention may also be applicable to other types of machines comprising at least one rotor having spiral lobes, for instance compressors of the so called single screw type and of the so called scroll type.
All the machines under consideration relate to such ones where the intermediate pressure port means is spaced from the main inlet port and disposed at such a distance therefrom that any communication therebetween through the working space of the machine is continuously blocked by at least one rotor lobe.
In order to vary the volumetric capacity of a screw compressor it is earlier known from U.S. Pat. No. 3,314,597, Schibbye, to provide the compressor with a selectively adjustable valve member controlling a bleed port in the wall of the working space so that a certain amount of the working fluid supplied to the compressor may be returned to the inlet channel of the compressor. This type of volumetric capacity control has been used also for screw compressors provided with intermediate port means. This bleed port is disposed within the same phase of the compression cycle as the intermediate port means. When the bleed port is opened the pressure level inside the compressor working space decreases to such an extent that the back pressure within the area of the intermediate port means will be practically the same as that in the low pressure channel. The bleed port must in order to avoid throttling losses be provided with a large area corresponding not only for the recirculation of the surplus fluid supplied through the inlet port but also for draining the fluid supplied through the intermediate port means. The size of the valve member will thus be too large for location in the end wall with regard to its area as well as the limited space available outside the rotor bearings. For this reason the valve has to be located in the barrel wall of the working space. Such a valve will consequently be complicated in shape and expensive to manufacture as it not only has to sealingly cooperate with its seat in the housing but also has to sealingly cooperate with the confronting rotor or rotors in order to avoid internal leakage in the compressor, especially when running under maximum capacity conditions.
The main object of the present invention is to achieve a more effective capacity control of the machine per se as well as of a complete plant by means of simpler and less expensive valve arrangements than those used in the prior art.